Wild-type D-serine deaminase (Dsdase, dsdA gene product) synthesis in Escherichia coli K12 is controlled by induction and catabolite repression, at the level of transcription. The induction control, which is mediated by the dsdC gene product, is strictly positive. The catabolite repression control is mediated by the cyclic AMP and cyclic AMP binding protein system. The operator locus, dsdO, is the site of action of both the induction and the catabolite repression controls. We are presently purifying the regulatory substance that mediates Dsdase induction, to characterize its interaction with the operator, and to obtain a detailed restriction map of the dsd region. Our ultimate aim is to define the mechanism by which interaction of the dsdC product and of the cAMP-cAMP binding protein with the promoter-operator effects transcription of dsdA.